Uroguanylin (UG) and guanylin (GN) are endogenous agonists of the guanylate cyclase C (GC-C) receptor that were discovered as natriuretic hormones based on their structural similarity to bacterial enterotoxin (ST), the secreted peptide that is responsible for traveler's diarrhea. Binding of these peptide hormones to the GC-C receptor stimulates intracellular production of cyclic guanosine monophosphate (cGMP), which in turn regulates ion and water homeostasis within the gastrointestinal mucosa. This group was the first to report that the expression of mRNAs encoding UG and GN are markedly reduced in cultured human colon carcinoma cells as well as human colon polyps and adenocarcinomas. In contrast, GC-C is overexpressed in human colon polyps and tumor tissues, perhaps to compensate for the reduced supply of UG and GN. The hypothesis of the proposed study is that near-infrared (NIR) fluorochrome-tagged SP-333, a stable analog of UG, can be used as a noninvasive probe for the reliable detection of spontaneous colorectal polyps in multiple intestinal neoplasia mice, in particular lesions that lack an elevated growth component. Preliminary data indicate that SP-333, a proteolytically resistant analog of UG, 1) possesses a high binding affinity for GC-C receptors; 2) is stable in simulated human gastric and intestinal fluids at 37oC; 3) can be tagged with an NIR fluorochrome; and 4) localizes preferentially to colorectal polyps, as compared to adjacent normal colonic mucosa, when incubated with mouse tissue ex vivo. These data support the further characterization and validation of NIR fluorochrome-tagged SP-333 as a specific probe for the detection of colorectal tumors in Dr. Clapper's strain of mice (Apc?FCCC) that uniquely develops multiple colorectal polyps. Specific Aim 1 will focus on determining the stability of NIR fluorochrome-tagged SP-333 in simulated intestinal and gastric fluids as well as the specificity of its binding to human carcinoma cells in vitro and excised colon tissue (normal and tumor). Once the optimal dose and schedule for probe administration have been established, its sensitivity and specificity to detect spontaneous colorectal polyps following oral administration to mice will be determined (Specific Aim 2). The proposed research will be accomplished jointly in collaboration with Dr. Clapper's group, who has recently established several imaging modalities for the detection of colorectal polyps in Apc?FCCC mice. Successful completion of the proposed research will have demonstrated the proof-of-concept that the GC-C agonist SP-333 can be used as a probe to reliably detect primary colon tumors, thus supporting its further development for translation to a clinical setting. Future evaluation of the utility of the probe to detect colorectal metastases may also be warranted.